Tim Marshall

Diplomatic Editor

Everyone around the table in Geneva knows the outline of the deal they may eventually achieve, but no one knows if they can fill in the details and get that far.

All previous negotiations between Iran and the six powers - the UK, US, France, Germany, Russia and China - have become bogged down.

Each time, the six powers feared that the Iranians had bought themselves another year working towards a nuclear weapon.

This time is different. There is a feeling that not only are the Iranians now serious about compromise, but that, if that analysis is wrong, there may not be any more negotiations, only more sanctions, and the threat of military action.

Iran is enriching uranium to 20% fissile purity and in quantities it does not require for peaceful purposes. From that level it is relatively easy to enrich it to weapons grade.

President Rouhani in New York last month

The six powers have long demanded that Iran suspends 20% enrichment and send its stockpile abroad. Now, it seems likely that the compromise offered will be to allow the existing stockpile to be held in Iran but that UN weapons inspectors must have unfettered access.

There would also be a requirement to allow inspection of lower-grade enriched uranium which can be refined to weapons grade.

The outside powers would require access to the Natanz enrichment plant (among others) where Iran has increased its number of centrifuges to 15,416.

US Secretary of State John Kerry with Israeli PM Netanyahu

In return, the six would accept Iran's right to enrich uranium, and equally importantly, ease and then lift the sanctions which have begun to really hurt the Iranian economy.

One diplomatic source said it was believed that the new President, Hassan Rouhani, won this year's election because enough Iranians thought he would take the measures required to lift sanctions and that this showed they were working.

However, all sides have limited room to manoeuvre. The hardliners in Tehran will want to see quick progress towards easing sanctions and some have already criticised their president's charm offensive at the United Nations in New York last month. The Western powers will want to see early signs of compromise by Iran - also known as "confidence building measures".

The Americans have to reassure the Israelis that they are not having the wool pulled over their eyes, especially after Israeli Prime Minister Benjamin Netanyahu's New York speech in which he called Mr Rouhani a "wolf in sheep's clothing".

One diplomat said: "We are not going into this starry eyed. There is a sense of a chance of deal, there's also a sense it might be the last chance."